This function is a quadratic whose coefficients are a=0.06, b= -7.2, c = 390.
Because a is +, the graph is a parabola that opens up and has a minimum. Your job is to find the value of b at this minimum (at the vertex). It is
b = -(-7.2) / [2(0.06)], or b = 60.
Production costs will be minimized if the number of bats made is 60.